Why does my plant roots melt in the tank?

[martinmin]

It's new tank. All the plants start with new and grows well in the first month. But now I noticed there are about 3 plants whose roots melted and the stems become black, dying, while other plants of the same type seem to grow well.

What cause these and how to avoid root melt?

 

[AllFishNoBrakes]

Are you sure the roots are melting? What plant(s) are you experiencing issues with?

As a general rule, you’re “buying the root system” and not the plant itself. Over time, the plant(s) should convert to your water and generally do pretty well. 
 

I’ve been in the hobby for 3+ years, and still haven’t found a Java Fern source/species that does well in my water, but other plants absolutely thrive. Might just take some trial and error to find what works best for you and your water structure/method of keeping plants and fish. 

[Mmiller2001]

Using a gravel substrate?